1. Field of the Disclosure
The present invention relates generally to energy transfer. More specifically, the present invention relates to an energy transfer assembly that may be utilized in a resonant power converter.
2. Background
Transformers are used in electrical circuits as energy transfer elements to transfer energy from an input electrical circuit to an output electrical circuit through the magnetic field with electrical isolation. In general, a transformer includes a primary winding and a secondary winding. The primary winding is connected to an input circuit and the secondary winding is connected to an output circuit. The primary and secondary windings are wound across the length of a common bobbin mounted on a magnetic core. Since the magnetic permeability of the magnetic core material relative to the surrounding air is much higher, the main flux transmitting the energy in the transformer is through the magnetic field in the magnetic core. However, part of the magnetic flux of each winding also passes through the air around the same winding, which is often referred to as leakage flux and results in a self inductance known as leakage inductance. In some situations the leakage inductance of the transformer is considered an unwanted parasitic inductance, while in other situations, the leakage inductance of the transformer is actually utilized as part of a required inductance for an electrical circuit design.
In known transformers, the primary and secondary windings are typically wound on top of each other with full overlap, and with each of the primary and secondary windings covering substantially the entire bobbin length, including any marginal isolation if required. In other known transformers, the primary and secondary windings are wound side-by-side along the bobbin length with each of them covering some percentage of the bobbin length, including any marginal isolation if required.
When the primary and secondary windings are wound with a fully overlapped structure, the majority of the magnetic flux generated by each winding is common between the windings, and the leakage inductance is therefore minimized. In addition, when primary and secondary windings are fully overlapped, due to the electrical potential difference between the adjacent or nearby layers of the windings, a parasitic capacitive (Cparasitic) coupling path is created between the primary and secondary windings, which forms a low impedance path between the primary and secondary windings for the common mode (CM) noise that typically has a noise frequency in the upper range of MHz (1/Cparasitic*ωnoise, where ωnoise=2πFnoise is the angular speed of the CM noise frequency). The CM noise travels simultaneously along both input lines and the path for their return to the source is through ground. The capacitive coupling between the primary and secondary provides a path for CM noise from the primary winding to the secondary winding and from the secondary through the parasitic capacitance of the secondary to ground through which the CM noise finds its way back to the input source. This causes an Electro Magnetic Interference (EMI) issue and may result in failures in Electro Magnetic Compliance (EMC) regulatory testing.
In transformers with primary and secondary windings that are wound in a side-by-side structure, the relative amount of the magnetic flux generated around each winding through the air is greater, and the leakage flux is increased compared to transformers with primary and secondary windings that are wound with fully overlapped structures. On the other hand, the capacitive coupling between the primary and secondary windings is minimized, which therefore results in minimized CM noise and EMI issues.
Switching converters often use transformers or other types of energy transfer elements. In HF switching converters, the transformer design is often a complicated task because as the frequencies of switching pulses increase, such as for example in the range of few hundred kHz, the role of transformer leakage impedance becomes more dominant (Lleakage*ωswitch, where ωswitch is the angular speed of the switching frequency).
In addition, CM noise is also a concern in switching converters that have HF input square wave pulses. The sharp edges found in the HF input square wave pulses can result in very high frequency (VHF) noise (angular speed of ωnoise) in the primary windings, which finds a return path through ground. For instance, in HF transformer isolated power supplies, the parasitic capacitive coupling between the winding layers forms a low impedance path (1/Cparasitic*ωnoise). CM noise with VHF noise is conducted through the path from the primary winding to the secondary winding. Furthermore, noise from secondary winding finds its way back to the input ground through the parasitic capacitance of secondary winding to ground, which creates EMI noise problems that appear in EMI scans as high amplitude spikes at the harmonics of the switching frequency and is a common cause of failure in EMC regulatory tests.